1. Field of the Invention
The invention relates generally to electrical control systems, and more particularly to systems and methods for controlling an electrical drive such as a variable speed drive of the type used in connection with electric submersible pumps used in downhole oil production, wherein the drive automatically determines the proper phasing to drive the pump motor in a forward direction and can then notify the operator or start the pump in the forward direction
2. Related Art
Crude oil is typically produced by drilling wells into oil reservoirs and then pumping the oil out of the reservoirs through the wells. Often, the oil is pumped out of the wells using electric submersible pumps. Electrical power is provided to electrical drive systems at the surface of the wells, and these drive systems provide electrical power to the pumps to allow them to pump fluid from the wells.
Electric submersible pumps can typically be operated in either “forward” or “reverse” directions. The forward direction, as that term is used herein, is the direction of rotation in which the pump is designed to pump fluid. This fluid is pumped up through the wellbore and out of the well. The direction of rotation opposite the forward direction is referred to herein as the reverse direction.
Many electric submersible pumps will still pump fluid out of the well when operating in the reverse direction, but they normally are not as efficient when pumping in the reverse direction as in the forward direction.
An electric submersible pump typically operates on three-phase power. The electrical drive system at the surface of the well normally receives AC power and converts this power to a three-phase drive signal that is conveyed to the pump motor to drive the motor, which in turn drives the pump to produce fluid from the well. When a pump is installed, the installation is performed according to procedures that are intended to be followed to ensure that the pump is properly connected to the electrical drive system. With the various junction boxes and cable splices between the drive and the pump, however, it is not unusual for mistakes to be made, resulting in electrical connections between the electrical drive system and pump motor that are incorrect. In particular, the cabling that carries the three-phase electrical signal from the electrical drive system to the pump motor may be connected with two or more of the wires switched. The misconnection of the wires in this cabling may also occur when maintenance is performed on the electrical drive system or the cabling.
Because the phasing of a three-phase electrical signal is reversed when any two of the three wires are switched, misconnection of these wires can result in the pump motor being driven in a direction which is opposite the intended direction. In other words, when the electrical drive system produces a drive signal with phasing that is intended to drive the pump in the forward direction, it actually drives the pump in the reverse direction. As noted above, while the pump may produce fluid from the well even in the reverse direction, this is not as efficient and does not produce as much fluid as driving the pump in the forward direction. Also, when the pump is run in the reverse direction, the resulting torque tends to loosen the connection between the pump and the tube string and the connections between individual pipe sections in the tube string.
It is assumed for the purposes of this disclosure that the phase differences between the three phases of the drive unit's output signals are substantially equal. When any two of the phases are switched, the effect is to reverse the order of the phases. For instance, if the phases on lines A, B and C occur in the order A-B-C, switching the signals on any two of the lines will result in the phase order C-B-A. It is therefore assumed that any output signal generated by the drive unit will have one of these two orders, or phasings.
When an operator is starting a downhole pump, but does not know the proper phasing for driving the pump in a forward direction, the operator must typically make an initial guess as to the proper phasing. The pump is then started using this phasing and is run for some amount of time which is sufficient to determine the production (the amount of fluid which is produced) using this phasing. Usually, the pump is operated until fluid is produced at the surface (the surface of the geological structure in which the well has been drilled). After the level of production has been measured with the pump running in this direction, the pump is stopped so that it can be run in the opposite direction. Normally, the column of fluid in the wellbore must be allowed to drain completely before the pump can be restarted in the opposite direction, which may require as much as several hours. The pump cannot normally be started until the column of fluid has drained from the well because the pump motor does not have sufficient torque to overcome the flow of the falling fluid, and trying to start the pump motor could cause it to be damaged. Once the fluid has drained from the wellbore, the operator runs the pump using phasing which is opposite the initial guess so that the pump is driven in the opposite direction. The pump is again operated for a period which is sufficient to measure the resulting production from the well. The pump may have to be run several times in each direction. The measurements corresponding to the different directions of rotation of the pump are then compared, and the direction which results in the higher production is assumed to be the forward direction. The pump is then restarted in the forward direction.
Although this procedure allows the well operator to determine the proper phasing to drive the pump in a forward rotational direction, it is not without its own problems. For instance, it is typically a very time-consuming process because it is necessary to operate the pump in both directions for long enough to produce fluid from the well in each direction. Additionally, it is necessary to wait for the column of fluid that has been established in the wellbore to drain back through the pump before the pump can be restarted in the opposite direction. All of this downtime during the procedure amounts to lost production from the well. This conventional procedure is also problematic because the steps of the procedure must be performed manually by the operator, which adds to the effective cost of the procedure and also presents the opportunity for mistakes to be made by the operator (e.g., mis-measurements of production amounts or mistakes in correcting the phasing).
It would therefore be desirable to provide systems and methods for automatically determining the proper phasing for driving a downhole pump in the proper (forward) direction and restarting the pump in the forward direction without the need for intervention by the well operator.